THE spy whose naked body was found in a sports bag in an empty bath was probably involved in an unusual sex game, police now believe.

Officers have ruled out almost every other possibility. They have come to the view that Gareth Williams probably died after climbing into the bag which was then locked by another person. It is unclear whether he did so on instructions from the other person or was locked in at his own request.

However, detectives believe that he was probably indulging in a sadomasochistic game in which he got a thrill from being helpless.

It is likely that once locked and left in the bag, he died from a combination of causes, including suffocation and dehydration -- difficult to identify in a post-mortem inquiry. The red holdall was made from a laminate material and had reinforced seams, making it hot and almost impossible to escape from.

Mr Williams's top-floor flat in Pimlico, central London, is likely to have been hot in the August weather, causing him to pass out.

The North Face bag was padlocked from the outside and officers believe another person was supposed to return to the flat to release him. But when they came back they found his dead body. Scotland Yard is still seeking a Mediterranean couple aged between 20 and 30, who were let into the spy's flat late one evening in June or July.